1. Field of the Invention
The present invention relates to an apparatus for estimating friction coefficients on road surfaces and particularly to an apparatus for estimating friction coefficients more accurately.
2. Discussion of Related Arts
In recent years, numerous vehicle control technologies such as a traction control technology, a braking force control technology, a torque distribution control technology and the like, have been proposed and some of these control technologies have been realized in actual automobile markets. Many of these control technologies use a friction coefficient on a road surface (hereinafter, referred to as road friction coefficient) for calculation or correction of control parameters. Accordingly, in order to make the control sure and precise, it is necessary to estimate accurate road friction coefficients.
There are several technologies in which road friction coefficients are estimated. For example, Japanese Patent Application Laid-open No. Toku-Kai-Hei 7-186989 discloses a technology in which a road friction coefficient is calculated according to a deviation of lateral acceleration. The deviation of lateral acceleration is a difference between an estimated lateral acceleration based on an actual yaw rate detected by a yaw rate sensor and an actual lateral acceleration detected by a lateral acceleration sensor.
Further, in Japanese Patent Application Laid-open No. Toku-Kai-Hei 8-2274 an inventor of the present invention proposes a technology wherein road friction coefficients are estimated based on the theory of adaptive control according to an equation of motion about a lateral motion of a vehicle using steering wheel angle, vehicle speed, yaw rate and other parameters.
However, Toku-Kai-Hei 7-186989 has a problem that the accuracy of estimation of road friction coefficient is easily influenced by resolution or noises of sensors. Accordingly, many conditions such as effective zero corrections, shield, a proper fitting position and the like are required from the sensors. Further, Toku-Kai-Hei 8-2274 has a disadvantage that since the principle of estimating road friction coefficients is based on a change of dynamic characteristics of vehicle motion according to the difference of road friction coefficients, it is difficult to detect road friction coefficients without vibratory steering inputs.
It is an object of the present invention to provide a road friction coefficient estimating apparatus which is capable of minimizing the influences of resolution, zero correction and noises of sensors, and which can estimate accurate road friction coefficients without vibratory steering inputs.
To accomplish the object, a road friction coefficient estimating apparatus for a vehicle according to a first aspect of the present invention comprises a reference value calculating means for calculating a reference value of the parameter, an actual value detecting means for detecting an actual value of a parameter indicative of a condition of motion of the vehicle, a deviation calculating means for calculating a deviation of the actual value from the reference value, a dispersion calculating means for calculating a dispersion obtained by statistically processing the deviation with a specified sampling number, and a road friction coefficient establishing means for establishing a road friction coefficient based on the dispersion. It is preferable to assign a yaw rate or a lateral acceleration to the parameter.
A road friction coefficient estimating apparatus for a vehicle according to a second aspect of the present invention comprises a momentary steering characteristic calculating means for calculating a steering characteristic at each moment based on an actual condition of motion of the vehicle, a dispersion calculating means for calculating a dispersion obtained by statistically processing the steering characteristic with a specified sampling number, and a road friction coefficient establishing means for establishing a road friction coefficient based on the dispersion.